Metal nanowires have been conventionally formed by a bottom-up ECD or electroplating through mask process, as shown in FIGS. 1A-1C. Specifically, a substrate structure comprising a supporting matrix 100 with one or more open pores 104 and a conductive base layer 102 is first provided, as shown in FIG. 1A. Each of the open pores 104 extends through the supporting matrix 100 onto the conductive base layer 102. An ECD or electroplating process is then carried out to deposit a metal material 106 over the conductive base layer 102 and to fill the open pores 104, as shown in FIG. 1B. After the open pores 104 are completely filled with the metal material 106, the ECD process is terminated, followed by selective removal of the supporting matrix 100, thereby forming free-standing metal nanowires 106, as shown in FIG. 1C. FIG. 2 is a picture of multiple metal nanowires formed by such a conventional bottom-up ECD process.
The conventional bottom-up ECD process as described hereinabove has also been used for forming compositionally modulated structures that comprise alternating layers of ferromagnetic materials and nonmagnetic materials, such as Co/Cu, Co/Ru, Co/Au, Ni/Cu, NiCo/Cu, NiFe/Cu, CoFe/Cu, FeCoNi/Cu, etc. Such compositionally modulated ferromagnetic-nonmagnetic structures are particularly useful in giant magnetoresistance (GMR) applications, which require alternating layers of ferromagnetic and non-magnetic materials.
However, the conventional bottom-up electrodeposition of the above ferromagnetic/nonmagnetic layered structures relies on the large difference between the reversible potentials of the ferromagnetic/nonmagnetic materials. In most cases, the nonmagnetic elements, such as Cu and Au, are much more noble than the ferromagnetic elements, such as Fe, Ni and Co. In other words, the nonmagnetic elements are electrodeposited at a much less negative potential than the ferromagnetic elements. In addition, the nonmagnetic and ferromagnetic elements do not interact with each other during electrodeposition. Therefore, an electrolyte with a small amount of nonmagnetic elements and an excess amount of ferromagnetic elements is generally used to form the ferromagnetic/nonmagnetic layered structures. At a relatively low negative potential, pure elemental nonmagnetic material is electrochemically deposited, while the ferromagnetic elements are not deposited. At a relatively high negative potential, both the nonmagnetic and the ferromagnetic elements are electrochemically deposited. Due to the small amount of nonmagnetic species available in the solution, the ferromagnetic elements are deposited at a much faster rate than the nonmagnetic elements, thereby resulting in a deposited layer with ferromagnetic characteristics.
The conventional bottom-up ECD process has never been used for forming compositionally modulated structures that comprise alternative layers of different ferromagnetic materials, which have very close reversible potentials and which may interact with each other during electrodeposition.